Income Taxes - Schedule of Reconciliation of the Federal Statutory Income Tax Rate and the Company's Effective Income Tax Rate (Detail)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	(21.00%)	(34.00%)	(34.00%)
State income taxes	(8.60%)	(6.90%)	(6.70%)
Research and development tax credits	(4.70%)	(3.40%)	(2.80%)
Stock-based compensation	(0.60%)	3.60%	1.90%
Change in U.S. tax rate		18.70%
Change in valuation allowance	34.90%	22.00%	41.60%

Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to changes in the valuation allowance for deferred tax assets.

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations applicable to state and local income tax expense (benefit), net of federal tax expense (benefit).

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